1. Field of the Invention
The present invention relates to a plotter using a knock-type pencil containing pencil-leads, and more particularly to a plotter with a system for sensing, by means of a reflection-type photosensor, a projection amount, from a pencil case, of a movable ring which holds a pencil-lead and is axially movable, thereby determining whether or not the length of the lead held by the movable ring exceeds an effective value for plotting (i.e. determining the "remaining/lacking" state of a pencil-lead).
2. Description the Related Art
With an X-Y plotter, a pencil for plotting is moved on the basis of plotting data output from a computer, thereby successively performing plotting. The pencil is, for example, a knock-type pencil containing a plurality of pencil-leads. One of the pencil-leads is successively fed, while the pencil is used. Thus, the pencil case includes a movable ring and a chuck mechanism for feeding the pencil lead.
The movable ring has a rubber tube for slidably holding the pencil-lead at its center. The movable ring is urged by a spring in such a direction as to move away from the chuck mechanism. The chuck mechanism holds and releases the lead fed from a knock mechanism. When the movable ring moves away from the chuck mechanism by virtue of the resiliency of the spring in the release mode of the chuck mechanism, the lead is fed from the chuck mechanism in accordance with the motion of the movable ring ("lead-feeding").
While the chuck mechanism holds the lead, the lead is not fed even if the movable ring tries to move by virtue of the force of the spring, as stated above. In addition, when a tip portion of the movable ring is abutted upon a lower knock plate (described later) and the movable ring is approached to the chuck mechanism against the urging force of the spring, the lead is immovable as long as the chuck mechanism holds the lead. Instead, the tip portion of the lead projects from the movable ring in the opposite direction by a degree corresponding to the motion of the movable ring.
By a single lead-feeding operation, drawings can be made until the movable ring projecting from the case to a maximum level retreats into the case to a maximum level. While the movable ring gradually retreats into the case, the lead is worn and shortened. In this case, vertical movement of associated parts is such that while the movable ring is abutted upon the lower knock plate and kept at a constant level, the pencil is gradually lowered by means of a up/down driving mechanism, thus gradually retreating the movable ring into the pencil. Accordingly, the amount of projection of the movable ring from the case is proportional to the amount of wear of the lead.
A reflection-type photosensor may be used to sense the amount of projection of the movable ring from the case. A detection light beam is emitted from an emission portion of the photosensor to a part of the movable ring, and a reflection beam is received by a receiving portion of the photosensor. On the basis of the amount of the received beam, the amount of projection is determined.
However, since the reflectivity of the surface of the movable ring varies depending on its cleanliness, the amount of received light may vary even if the amount of projection is unchanged, and this may result in a malfunction (erroneous determination). The reflectivity of the surface may vary due to not only time-base change of surface condition but also rotation of the movable ring in its circumferential direction. Furthermore, the amount of received light may vary due to presence/absence of ambient disturbance light, and this may also result in a malfunction.